...and yet they're still covered by a huge heatsink.

To add perspective, an old-school 7805 voltage regulator dissipating just 1 watt is already impossibly hot to hold with bare hand (as me how I know). So 3-4 watts on a small module will make it noticeably hot.

They aren't huge at all, the new RTL cards are tiny. I wish 2-port versions were available for a home server upgrade.